Heretofore, screening devices have been provided which operated generally horizontally but with a strictly gyratory motion to distribute the product over the entire screen surface and some devices included bouncing balls, or the like, which were intended to prevent plugging of the screen. The balls were caused to bounce against the screen by the motion of the machine. These machines primarily comprised devices which relied upon brute force to achieve their purpose and consequently consumed more power and were expensive to operate.
To change the "stroke" of such prior machines it was necessary to deenergize the machine and the rotating eccentric weights then altered to modify the stroke, whereas customers wanted a low power consuming machine having a flat stroke screen which was readily adjustable as to the stroke and the frequency control. Such conventional screening apparatus utilized single power means and the component input to the screen resulted in similar stroke actions.